Makefile: add -Werror=discarded-qualifiers to the C flags

Because C allows this:

  void f(int *x);
  void g(const int *x)
  { f(x); }

Signed-off-by: Thiago Macieira <[email protected]>

Author: thiagomacieira

Makefile

@@ -240,6 +240,7 @@ cflags += -std=gnu99 $(CFLAGS)
 
 ifneq ($(DISABLE_WERROR),1)
 cflags += \
+	-Werror=discarded-qualifiers \
 	-Werror=incompatible-pointer-types \
 	-Werror=implicit-function-declaration \
 	-Werror=int-conversion